They're cute crabs!
Okay, so this is a little off-topic but your sand looks really dry. I just had this happen yesterday in my own tank, too. I hate winter! Make sure it's sand-castle consistency all the way through so that they can bury to molt when it's time. It'll also really help your humidity levels. That's how I knew my tank was in trouble, because my humidity tanked all of a sudden.
Is it playsand? It looks really yellow like colored calcium sand, but that could be your camera.
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